We propose to use the neural system controlling song in passerine birds as a model system to study several interrelated issues. Using systemic treatment and intracranial microimplants of sex steroids, we propose to study cellular mechanisms by which sex steriods influence neuronal growth and differentiation at various periods in life. This involves light microscopic measurements of morphology of neurons and of the sizes of brain regions as a function of various steroid hormone treatments. We propose to examine what role (a) androgenic steroids have in (b) neural changes related to (c) vocal learning, by measuring these three (a, b, c) variables as they change in several species of birds. We propose to study sexual differences in neural morphology in several species which have sex differences in vocal behavior, to understand better the relationship between neural morphology and behavior, and to understand better the neural bases of sex differences in behavior. We propose to study the role of sensory feedback in song learning. These experiments will increase our understanding of sex hormone actions on the brain during development, and provide insight into the relationship between brain structure and behavior.